Biomedical signal acquisition has gained much attention in recent years due to the fast growing market for portable biomedical electronics such as wearable or implantable health monitoring devices. Subthreshold-mode CMOS amplifiers with integrated band-pass functions are often used in such devices to achieve high power efficiency. In order to effectively reject the DC component and baseline drift without distorting the useful bio-signal, such amplifiers need to provide very low high-pass cut-off frequencies (sub 1 Hz). Along with onchip capacitors, MOS-bipolar pseudo-resistors that produce up to 1013 ohm or even higher resistance are usually employed to realize the said high-pass function, while avoiding the use of external RC components. However, the process dependence and highly nonlinear resistance associated with pseudo-resistors may cause problems such as metastable operational points and unpredictable corner frequencies.
FIG. 1 depicts a typical neural recording amplifier 100 with integrated band-pass function. Assuming that the operational transconductance amplifier (OTA) 102 is ideal, the gain of the overall system is set by the capacitance ratio C1 112/C2 114 (or C3 116/C4 118); the input DC component is blocked by the high-pass network C1 112, M1 122 and M2 124 (or C3 116, M3 126 and M4 128), whose corner frequency is regulated by C2 114, M1 122 and M2 124 (or C4 118, M3 126 and M4 128). Once the values of these components are determined, the gain and the high-pass corner frequency are fixed. However, they may drift considerably against the design target due to process variations, yet no tuning and adjustment are possible. Furthermore, the resistance provided by the active pseudo-resistors M1 122 and M2 124 (or M3 126 and M4 128) is asymmetric, which varies when the voltage drops across the pseudo-resistor structure 132 (or 134) are equal in magnitude but opposite in direction. This leads to signal-dependent output drift and degradation in linearity. As the input level increases, the output usually encounters a premature clipping at one of the power rails, which severely degrades the dynamic range.
A few attempts have been made to improve the linearity and dynamic range of such amplifiers. Some of them incorporate features such as tunable bandwidth and programmable gain. Listed below are typical examples of such attempts.    [1] H. Wu and Y. P. Xu, “A 1V 2.3 μW Biomedical Signal Acquisition IC,” Proceedings of the 2006 IEEE International Solid-State Circuit Conference (ISSCC), pp. 119-128, February 2006;    [2] R. R. Harrison and C. Charles, “A Low-Power Low-Noise CMOS Amplifier for Neural Recording Applications,” IEEE Journal of Solid State Circuits (JSSC), Vol. 38, No. 6, June 2003;    [3] M. Yin and M. Ghovanloo, “A Low-Noise Preamplifier with Adjustable Gain and Bandwidth for Biopotential Recording Applications,” Proceedings of IEEE International Symposium on Circuits and Systems (ISCAS), pp. 321-324, May 2007;    [4] W. Wattanapanitch, M. Fee and R. Sarpeshkar, “An Energy-Efficient Micropower Neural Recording Amplifier,” IEEE Transactions on Biomedical Circuits and Systems, Vol. 1, No. 2, June 2007.
Although these works have been helpful in correcting certain aspects of the problem, they usually show negative effects on others. For example, enhancing the linearity of active loads may lead to prolonged settling time and diminished possibility for post-fabrication adjustment; on the other hand, incorporating tunability often introduces increased severe imbalance across the structure and produces reduced linearity and dynamic range.
Therefore, there exists a need to provide a low voltage CMOS amplifier with integrated tunable band-pass function, a tunable active resistor structure, a method of amplifying an input signal and a method of fabricating an amplifier which seek to address one or more of the problems mentioned above.